jan 1, 1868 - Joseph Stearns invented the duplex.
The duplex was a system that transmitted two messages over a single wire at the same time. This was the invention that started the push to invent the actual telephone in which voices could be transmitted. After the duplex was invented, Stearns hired Thomas Edison to make advancements on it and he managed to create the quadruplex but they still wanted more lines. So they hired Alexander Graham Bell and he got it up to ten lines. But this invention started Bell in his journey to invent the telephone.
Added to timeline:
The Atom vs the Telephone